Hitler was also a vegetarian, which he abruptly became following the suicide of his neice (whom he was rumored to have fancied). Albert Einstein was also a vegetarian, and a Jew. Hitler hated Jews (actually, he wanted to have them all killed or at least out of areas governed by the Third Riech, if you can believe that--some don't). But if Hitler and Einstein had dinner, they would for sure not have meat.

Lots of prominent Americans (including Lindbergh) around the time that There Will Be Blood takes place in thought that Hitler was a great and visionary leader for having pulled Germany out from its economic depression. Perhaps the Day-Lewis character would have felt the same way, and even modelled his own facial hair after "der Führer." Of course, Hitler's American supporters did not yet realize that Hitler wanted to kill all of the Jews.

Hitler himself was Austrian, not German, and his "Aryan" heritage was questionable.